Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process.
One problem which arises in practice is that, in particular as a result of the absorption of the radiation emitted by the EUV light source, the EUV mirrors heat up and thus undergo an associated thermal expansion or the deformation, which in turn can negatively affect the imaging properties of the optical system. This is the case in particular if illumination settings with comparatively small illumination poles are used (e.g. in dipole or quadrupole illumination settings), in which the mirror warming or deformation varies strongly over the optically effective surface of the mirror.
Variations in gravity in dependence on the placement site or the geographic location of the system, for example, are another cause of aberrations occurring during aberration of a projection exposure apparatus.
It is in particular known, for at least partially compensating the above-described problems and also generally for increasing the image position accuracy and image quality (both along the optical axis, or in the light propagation direction, but also in the lateral direction, or perpendicular to the optical axis or light propagation direction), to design one or more mirrors in an EUV system as an adaptive mirror with an actuator layer made of a piezoelectric material, wherein an electric field of locally varying strength is generated across the piezoelectric layer by applying an electric voltage to electrodes arranged on both sides of the piezoelectric layer. In the case of a local deformation of the piezoelectric layer, the reflection layer stack of the adaptive mirror also deforms, with the result that (possibly also temporally variable) imaging aberrations can be compensated for at least partially by appropriately controlling the electrodes.
FIG. 4 shows a construction of a conventional adaptive mirror 30, which is possible in principle, in a merely schematic illustration. The mirror 30 having the optically effective surface 31 has between a mirror substrate 32 and a reflection layer stack 41 (for example as a multilayer system made of molybdenum and silicon layers) a piezoelectric layer 36 which is produced from a piezoelectric material, such as for example lead zirconate titanate (Pb(Zr,Ti)O3). The mirror substrate material can be, for example, quartz glass doped with titanium dioxide (TiO2), with examples of materials that are usable being those sold under the trade names ULE® (by Corning Inc.) or Zerodur® (by Schott AG). The piezoelectric layer 36 is arranged between a first electrode 34, which according to FIG. 4 is applied to an adhesive layer 33 (in the example made of TiO2) provided on the mirror substrate 32, and a second structured electrode 38, wherein another adhesive layer 35 and 37 (in the example made of LaNiO3) is disposed between the electrodes 34 and 38 and the piezoelectric layer 36. The adhesive layer 35 and 37 serves to make available crystalline growth conditions for the piezoelectric layer that are as optimal as possible.
According to FIG. 4, a screening layer 40 (which in the example is made from platinum (PT) just like the electrodes 34, 38 and which is optional in principle) is furthermore disposed on the bottom side of the reflection layer stack 41, which faces the structured electrode 38. According to FIG. 4, a SiO2 layer 39 is furthermore disposed between the piezoelectric layer 36 and the screening layer 40. By applying a locally varying electric voltage, a locally varying deflection of the piezoelectric layer 36 can be produced, which in turn converts into a deformation of the reflection layer stack 41 and thus into a wavefront change for light that is incident on the optically effective surface 31 and which can be used for aberration correction.
Even though the above-described principle of an adaptive mirror makes efficient aberration correction in conjunction with the deformation or actuation of the mirror 30 possible to some degree, the requirement of greater actuations or deformations brings about the problem that the displacement distances that are realizable through deflection of the piezoelectric layer are limited in principle.
One reason for this limitation of the realizable displacement distances is based on the principle thickness limitation of the piezoelectric layer, which is a result of the fact that the piezoelectric material grows on a crystalline structure which is no longer sufficiently perfect if a specific thickness (which for example in the case of lead zirconate titanate as the piezoelectric material can be approximately 2 μm) is exceeded, which ultimately results in a reduction of the so-called “d33 coefficient” that is characteristic of the voltage-induced expansion of the piezomaterial and thus also in a decrease of the actuation effect for the deformation of the mirror. The d33 coefficient is here defined byΔD=d33*U  (1),ΔD designating the (absolute) thickness change and U designating the electric voltage.
Furthermore, an increase of the electric voltage applied to the electrodes in the region of the piezoelectric layer, which increase is likewise to be considered for realizing greater displacement distances, is likewise subject to limits, which is due both to hysteresis effects occurring in the piezomaterial when stronger electric fields are applied or electric voltages of more than 20 V are applied, and to damages to the piezoelectric layer in case the electric voltage increases too much and a resulting decrease in service life.
Regarding the prior art, reference is only made by way of example to DE 10 2011 081 603 A1.